1. Field of the Invention
The invention relates to an arrangement of surfaces with an optical diffraction effect, as set forth in the classifying portion of claims 1 and 7.
Surface patterns of that kind are additionally used to enhance the level of safeguard against forgery in relation to optical-diffraction security elements which are made up of holograms and/or diffraction gratings of a mosaic-like composition.
2. Description of the Prior Art
An arrangement of the kind set forth in the opening part of this specification, in relation to optical-diffraction security elements, is known from EP-A 105 099 and EP-A 375 833. EP-A 105 099 describes the production of variable patterns of surface portions which have an optical diffraction effect and which are glued for example in the form of a stamp on a document and authenticate that it is genuine. When the security feature is illuminated, those surface portions are successively lit up upon rotation about an axis perpendicularly to the plane of the security feature, along a path. EP-A 375 833 describes a security element whose surface is divided into grid fields or areas and each grid field or area is subdivided into a number of field portions. The number of field portions per grid field determines the number of motifs or images which are successively visible at predetermined viewing directions. The same field portions of all grid fields form the picture elements (=pixels) of one of the images or motifs and they have such grating structures that the image or motif is visible only from one predetermined direction. The brightness of a pixel of the image is predetermined by the surface proportion of the diffraction structure in the field portion.
It is also known from EP-A 0 360 969 for at least one optical diffraction element of the optical-diffraction authenticity feature to be subdivided into two surface portions whose microscopically fine asymmetrical diffraction gratings only differ in terms of azimuth through 180xc2x0, with the other grating parameters otherwise being the same. With those subdivided diffraction elements, machine-readable information can be inconspicuously disposed in a visually perceptible pattern.
U.S. Pat. No. 5,059,776 shows a barcode area which is divided into bar elements and background elements, according to the bar coding. The surfaces of all bar elements are occupied by a first asymmetrical relief profile and the surfaces of all background elements are occupied by a second asymmetrical relief profile. The sequence of wide and narrow bar elements is determined by the information in the bar code area.
The above-listed documents describe security elements without effectively utilising brightness modulation within relatively large areas which can be well perceived by the naked human eye.
In addition EP-A 0 401 466 describes a plastic laminate with embedded, microscopically fine, optically effective relief structures and the use thereof as a security element. The materials which can be used are known for example from EP 0 201 323 B1.
The object of the invention is to provide an inexpensive surface pattern which is difficult to forge even with holographic copying processes, with a new authenticity feature which is readily visible in diffuse light, for optical-diffraction security elements.
In accordance with the invention that object is attained by the features recited in the characterising portions of claims 1 and 7. Advantageous configurations of the invention are set forth in the appendant claims.
Embodiments of the invention are described in greater detail hereinafter and illustrated in the drawings.